Conventional solid-oxide fuel cells utilize an electrolyte located between an anode and cathode facilitating the transfer of ions therebetween. Traditionally, solid-state fossil fuels, such as coal, had to be gasified and reformed prior to being introduced to a solid-oxide fuel cell to generate electric energy. Despite the additional processing steps required, the use of such solid-state fossil fuels to generate electricity remained an attractive option due in part to the high energy density of such fuels. The separate gasification and reformation steps required a substantial influx of thermal energy, and heat recovery was low, thereby resulting in an inefficient process.
Attempts to improve process efficiency for generating electricity with a solid-oxide fuel cell included the incorporation of the reformation step into the fuel-cell apparatus, which required to fuel cell to operate at temperatures above 1473 K. By incorporating the reformation step within the fuel cell, at least a portion of the heat required to reform the fossil fuel could be recovered and utilized during the electrical-energy generation process. However, such fuel cells experienced a buildup of fly ash, a byproduct produced from the combustion of solid fossil fuels, on the surface of a catalyst provided to an electrode, and accordingly, a retardation of the fuel cell's performance. It is believed that the buildup of fly ash was due to the elevated operating temperatures of the fuel cell, greater than about 1473 K, which is greater than the fly ash fusibility temperature that must be exceeded before the fly ash can deposit on the catalyst's surface. Additionally, these conventional, high-temperature fuel cells also exhibited excess sulfur precipitation on the catalyst surface and high NO emissions.
Alternate designs have been proposed to convert solid-state fossil fuels directly to CO2 and electric energy while minimizing the production of CO. Such designs divided the fuel cell into a plurality of different temperature zones, and dedicated a heating element for controlling the temperature in each respective temperature zone. Both fuel-cell electrodes, one on each opposing side of a solid electrolyte, were formed from either the same noble-metal or the same mixed conducting oxide material to facilitate the complete oxidation of carbon according to the reaction:C+O2→CO2 Such designs were cumbersome due to the requirement of different temperature zones to provide a sufficiently-high temperature to minimize the impedance to ion-conduction by the solid electrolyte, while at the same time providing a sufficiently-low temperature to favor the complete oxidation of carbon to CO2.
Other conventional fuel-cell designs have attempted to generate electrical energy from fossil fuels without providing a plurality of different temperature zones. As previously mentioned, solid-state fossil fuels such as coal can be introduced into a fuel cell having gasification features for gasifying the solid-state fuel before the carbon is oxidized to generate CO, H2 and electric energy. Electrodes made from different materials are installed adjacent to the solid-oxide electrolyte to generate ions and facilitate the partial oxidation of the fossil fuel according to the reaction:
                    C        m            ⁢              H        n              +          m      ⁢                          ⁢              O                  -          2                      →            m      ⁢                          ⁢      CO        +                  n        2            ⁢              H        2              +          2      ⁢      m      ⁢                          ⁢              ⅇ        -            Such fuel cells prevent the favoring of complete oxidation of the fossil fuel, even after gasification, to produce CO2 by requiring an excess of carbon at the oxidation electrode.
Accordingly, there is a need in the art for a fuel cell that can produce electric as a product of a reaction that directly and completely oxidizes a solid-state fossil fuel to produce CO2. The fuel cell should minimize costs and inefficiencies associated with a gasification step prior to being consumed according to the direct oxidation reaction. Further, the fuel cell should address the competing temperature requirements of the electrolyte impedance and the direct oxidation reaction to favor production of CO2 over CO.